With today's risk of global climate change and food shortage, finding ways to improve CO2 uptake by terrestrial plants is becoming an increasingly important concern. Stomata, the principal organs in CO2 uptake, are tiny pores surrounded by a pair of special cells (called guard cells) on the epidermis, and are primarily found on the leaf surface of terrestrial plants. Because the leaf surface allows almost no passage of air and water, stomata provide the principal pathway for diffusion of CO2, O2 and water vapor between ambient air and the leaf interior. Promotion of gas exchange by opening the stomata is one of the most crucial processes in plant photosynthesis and transpiration (1, 2). Recent studies have shown that stomatal transpiration is the limiting factor for photosynthesis in rice plants (3), and that a decrease in transpiration brings about a drop in nutrient absorption in crops (such as wheat) (4). Therefore, it is expected that an increase in the stomatal aperture and thereby the volume of transpiration will promote photosynthesis, leading to increased plant growth. However, as far as the present inventors know, there have been no reports of studies which have succeeded in increasing the opening of the stomata in order to achieve increased plant growth. One reason for this is believed to be that since the stomata also play the role of sluice gates (5), it is difficult to simultaneously balance the loss of water vapor via the stomata and the uptake of CO2.
Light is one of the primary factors that stimulate the opening of the stomata, and a variety of mechanisms form the basis for the opening of the stomata in response to various wavelengths of light (6-8). Red light is thought to induce the opening of the stomata as well as a reduction in the concentration of CO2 in the intercellular space (Ci) via photosynthesis which takes place in the mesophyll and guard cell chloroplasts (5, 9, 10). However, the detailed mechanism of stomatal response to red light is under debate (11, 12). In contrast, blue light acts as a signal and exhibits the most marked effect on stomatal opening. Blue light receptor phototropins (phot1 and phot2) activate cell membrane H+-ATPase via phosphorylation of the second threonine from the carboxyl terminus and subsequent binding of 14-3-3 protein to phosphothreonine (13-15). Cell membrane H+-ATPase activated by blue light induces hyperpolarization of the cell membrane, thereby enabling uptake of K+ via inwardly rectifying K30  channels (cell membrane K+in channels) (16-20). Accumulation of K30  induces expansion of the guard cells and causes the stomata to open. Therefore, these three factors (phototropins, cell membrane H+-ATPase, and cell membrane K+in channels) play an important role in stomatal opening due to blue light. In addition to these factors, it has been suggested that the FLOWERING LOCUS T (FT) is a positive regulator of stomatal opening (21).